Why is there no sound from the headphones when connected to the USB-A port on GC313 or GC313Pro?

Ensure that the signal source has audio and that the provided Type-C to Type-C cable is connected to the C1 port of the GC313Pro.
Additionally, please check whether the headphones connected to the A port of the GC313Pro are recognized by the signal source device (e.g., your computer) connected to the C1 port as an audio output device.
If you are using the GC313Pro, please note that the headphones can only receive audio from the signal source connected to the C1 port of the GC313Pro.